Fluid property sensor with heat loss compensation and operating method thereof

ABSTRACT

An improved Pirani sensor uses a measuring element disposed within a fluid between a base plate and a cover. The measuring element is held by suspension members that are connected to the base plate. A heating element is thermally conductively connected to the suspension members. Using the sensor the characteristic of the fluid is determined by evaluating the heat transfer from the thermal element through the fluid into the cover when heating power is applied to measuring element. Parasitic conductive heat loss from the measuring element into the suspension members is compensated by applying power to the heating element.

TECHNICAL FIELD

The present disclosure generally relates to thermal sensors formeasuring fluid characteristics which change with thermal conductivityand heat capacity of a fluid, and more particularly, to Pirani sensorsfor measuring gas pressure.

BACKGROUND

A Pirani sensor consists of a measuring element suspended in a tubewhich is connected to the system whose vacuum is to be measured. Themeasuring element is typically a heated metal wire (also called afilament). A filament suspended in a gas will lose heat to the gas asits molecules collide with the wire and remove heat. If the gas pressureis reduced the number of molecules present will fall proportionately andthe wire will lose low heat more slowly. Measuring the heat loss is anindirect indication of pressure. The filament is connected to anelectrical circuit from which, after calibration, a pressure reading maybe taken.

Exemplary Pirani sensors are disclosed in German patent no. DE19903010and in European patents No. EP0660096 and EP1409963. Further exemplarysensors and their operating modes are disclosed in U.S. Pat. Nos.7,360,415; 7,497,118; 7,642,923 and 8,047,711 which are herebyincorporated herein by reference in their entireties.

While the heat loss from the filament into the gas is an indicator ofthe gas pressure, conventional Pirani sensors also experience conductiveheat loss from the filament into the filament's suspension and radiationheat losses from the filament. During operation the conductive heat lossinto the suspension (P_(suspension)) and radiation heat loss(P_(radiation)) add up to a base power P_(zero) which is required tomaintain the operating condition of the sensor. This base power may alsobe referred to as “zero pressure” p₀, indicating the pressure that wouldlead to the same heat loss into the gas as the parasitic effects ofconductive and radiation heat loss in a complete vacuum.

The base power P_(zero) of a Pirani sensor depends on the sensor'sgeometry, material properties, and environmental conditions in which thesensor operates, especially the ambient temperature. The materialproperties that affect base power include the emission coefficient ofthe measuring element (filament) surface and reflection properties ofthe surface into which power is transferred by radiation. While theinfluence of the geometry, for example the thickness of the heatdissipating suspension pins, and the influence of the materialproperties may be assumed to be design related constants, the influenceof the ambient temperature is variable.

The greater the amount of parasitic heat losses and therefore the basepower P_(zero), the more difficult the detection of small changes in thethermal conductivity, heat capacity, pressure or flow of the measuredfluid becomes.

Goal in the design of such sensors is therefore a minimization of basepower P_(zero). For reasons of mechanical stability, however, there is atight limit to reducing the dimensions of the measuring element'ssuspensions. The suspension must be capable of carrying the measuringelement, which may be a (metal) wire, measuring filaments, or a membranethat carries measuring elements.

Conventional approaches have attempted to compensate for changingambient influences, such as ambient temperature, by additional measuringresistors in the electrical evaluation circuit to which the sensor isconnected. Those approaches are, however, limited. Since the ambientinfluence on a Pirani sensor depends not only on the temperature butalso on the pressure of the measured fluid, compensation by atemperature sensor is, strictly speaking, valid only for a singleoperating point.

SUMMARY

The present disclosure provides an improved thermal conductivity sensorfor measuring a characteristic of a fluid which substantially reducesthe parasitic heat loss through the measuring element's suspension andradiation heat losses from the measuring element. The reduced heatlosses result in a reduced base power P_(zero) and zero pressure p₀. Thereduced base power P_(zero) improves the signal to noise ratio of thesensor, and correspondingly the sensor's accuracy. The improved sensoralso reduces or eliminates surface contamination of the measuringelement, or reduces its effect on the measurement.

The improved sensor uses a measuring element disposed within a fluidadjacent to a heat sink. The measuring element may for example be ametal wire, a filament, or a flat meander-shaped metal foil. Themeasuring element is held by suspension members. The suspension membersmay be connected to a base plate. The suspension members may for examplebe electrically conductive suspension pins or be a silicon structure incase of a micro-Pirani sensor chip. A suspension heating element isthermally conductively connected to the suspension members. Thesuspension heating element may be a heating resistor. Alternatively, twosuspension heating elements may be used, one each thermally conductivelyattached to two suspension members.

The thermal conductivity sensor may be used to determine the flow rateof a fluid across the sensor, or to identify a fluid based on itsthermal conductivity. An important application of the sensor is its useas a Pirani sensor to measure gas pressure in a vacuum. When used as aPirani sensor, pressure of the gas around the sensor is determined byevaluating the heat transfer from the thermal element through the gasinto the adjacent heat sink when heating power is applied to measuringelement. While measuring the heat transfer through the gas, a Piranisensor traditionally experiences parasitic conductive heat loss from themeasuring element into the suspension members. In the here presentedsensor this parasitic heat loss is compensated by applying compensationpower to the suspension heating element. The compensation power ispreferably chosen such, that the suspension members are heated toapproximately the same temperature as the measuring element. Thecompensation power may partially reduce or completely eliminateparasitic suspension heat loss. The compensation power may e.g. reducethe parasitic suspension heat loss by more than 90%.

In another application the characteristic of the liquid or gaseous fluidthat is to be measured may be the fluid's flow rate. While the conceptpresented in this paper is primarily based on use of a heating elementthat is thermally conductively connected to the suspension members,temperature control of the suspension members may also be based on useof a cooling element, for example a Peltier element. This may be usefulin special applications such as measuring flow in cold fluids.

The heat sink is positioned near the measuring element and serves as aheat exchange surface to enable conductive heat flow through the fluidthat is to be measured. The heat sink may simultaneously serve as acover of the sensor and protect the measuring element from damage andcontamination. To control the temperature of the heat sink it may bethermally conductively connected to a heat sink heating element.

The sensor is controlled by a control processor, which is operativelyconnected to the measuring element and to the suspension heatingelement. The control processor is configured to determine thetemperature of the measuring element. The control processor may also beconfigured to determine the temperature of the heating element. Variousoperating methods exist. In a preferred method the control processorapplies power to the heating element to maintain a substantiallyconstant temperature T₁. The control processor further applies pulsedpower to the measuring element until the measuring element reaches apredetermined temperature T₂.

The sensor may be calibrated by exposing it to an ultra-high vacuum,ideally at or below its lower sensing range. During calibration power tothe suspension heating element is adjusted, until the voltage across abridge circuit into which the measuring element is connected reaches alower threshold. Calibration of suspension heating may be based onadjusting a variable resistance which is operatively connected to asuspension heating control element. Alternatively, calibration may bebased on storing a value in non-volatile memory, which is used by thecontrol processor to control the suspension heating element.

The measuring element may be connected to an electronic circuit whichcomprises an upper threshold comparator operatively connected to thecontrol processor for detecting an upper temperature threshold of themeasuring element. The control processor applies, through a variablevoltage generator, power to the measuring element until a signal fromthe upper threshold comparator is received. The control processor thenturns off or substantially reduces the output of the variable voltagegenerator for a predetermined time t_(w) after the signal from the upperthreshold comparator is received

Instead of letting the measuring element cool down for a predeterminedtime t_(w), cooling of the measuring element may be based on letting itcool to a predetermined intermediate temperature. This is accomplishedby the addition of an intermediate threshold comparator operativelyconnected to the control processor for detecting an intermediatetemperature threshold of the measuring element. In this case the controlprocessor applies, through a variable voltage generator, power to themeasuring element until a signal from the upper threshold comparator isreceived, and turns off or substantially reduces the output of thevariable voltage generator until a signal from the intermediatethreshold comparator is received

A beneficial operation of the sensor is achieved when T₁ is about 60°C.-120° C. and T₂ is about 80° C.-140° C. T₂ should be selected to beabout 20° C. higher than T₁, beneficially between 10° C. and 40° C.higher than T₁.

A method for measuring a characteristic of a fluid comprises disposing ameasuring element within a fluid, the measuring element being held bysuspension members. Measuring power is applied to the measuring element,either constantly or in pulses. If pulses are used to power themeasuring element those may be of constant voltage or current, orfollowing a characteristic curve, e.g. a voltage or currant ramp with aknown ramp angle α. In pulse operation measuring power is applied untilthe measuring element reaches an upper temperature of T₂.

Compensation power is applied to one or more suspension heating elementswhich are thermally conductively connected to the suspension members.The compensation power is selected to compensate, at least partially,parasitic conductive heat loss from the measuring element into thesuspension members.

The heat transfer from the thermal element through the fluid to thecover by measuring the power applied to the measuring element isevaluated, and a characteristic of the fluid is derived. Evaluation ofthe heat transfer from the thermal element into the fluid is achieved bymeasuring a time t_(x) required to heat the measuring element from afirst temperature T₁ to a second temperature T₂. Subsequently acharacteristic of the fluid is derived by calculating a measure from thetime t_(x).

In pulsed operation, following the heating of the measuring element totemperature T₂ power is removed, allowing the measuring element to cooldown. Following the application of measuring power a predetermined waitperiod t_(w) may be applied until a subsequent application of measuringpower, and the frequency 1/(t_(x)+t_(w)) may be used to derive thedesired characteristic of the fluid. Other mathematical calculationsbased on t_(x) and t_(w) may also be used.

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses thereof.Furthermore, there is no intention to be bound by any theory presentedin the preceding background or the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side sectional view of a prior art Pirani sensor.

FIG. 1B is a top sectional view A-A of the sensor as in FIG. 1A.

FIG. 2A is a side sectional view of a prior art micro-Pirani sensor.

FIG. 2B is a top sectional view A-A of the sensor of as in FIG. 2A.

FIG. 3A is a side sectional view of a Pirani sensor with a suspensionheating element for loss compensation.

FIG. 3B is a top sectional view A-A of the sensor as in FIG. 3A.

FIG. 3C is a schematic diagram comparing the operating characteristicsof sensor with and without heat loss compensation.

FIG. 4A is a side sectional view of a micro-Pirani sensor withsuspension heating elements for loss compensation.

FIG. 4B is a top sectional view A-A of the sensor as in FIG. 4A.

FIG. 5A is a top sectional view of an alternative Pirani sensor with asuspension heating element for loss compensation showing a filamentmeasuring element.

FIG. 5B is a side sectional view B-B of the sensor as in FIG. 5A.

FIG. 5C is a front sectional view A-A of the sensor as in FIG. 5A.

FIG. 5D is a top sectional view of an alternative Pirani sensor with asuspension heating element for loss compensation showing ameander-shaped measuring element.

FIG. 5E is a side sectional view B-B of the sensor as in FIG. 5D.

FIG. 5F is a front sectional view A-A of the sensor as in FIG. 5D.

FIG. 5G is a detailed view of a meander-shaped measuring element.

FIG. 6A is a front sectional view of an alternative Pirani sensor with acover thermally conductively connected to a base plate.

FIG. 6B is a front sectional view of an alternative Pirani sensor with asuspension heating element and a heat sink heating element.

FIG. 6C shows an electronic circuit for operating a Pirani sensor withheat loss compensation in continuous mode.

FIG. 6D shows an alternative electronic circuit for operating a Piranisensor with heat loss compensation in continuous mode.

FIG. 7A is a top sectional view of a micro-Pirani sensor mounted onto aheating element.

FIG. 7B is a side sectional view of the micro-Pirani sensor as in FIG.7A.

FIG. 7C is a side sectional view of an alternative micro-Pirani sensorwith heat loss compensation.

FIG. 8 is a cross sectional view of an alternative micro-Pirani sensorwith a suspension heating element.

FIG. 9A is an exploded view a Pirani sensor with heating elementenclosed by a metal bracket.

FIG. 9B is a cross sectional view of the Pirani sensor in FIG. 9A.

FIG. 10A is a diagram to illustrate temperature and voltage of a Piranisensor over time when used in an electronic circuit as in FIG. 10D.

FIG. 10B is a diagram to illustrate heating pulses applied to the Piranisensor as in FIG. 10A.

FIG. 10C is a diagram to illustrate alternatively shaped heating pulsesapplied to the Pirani sensor as in FIG. 10A.

FIG. 10D shows an electronic circuit for operating a Pirani sensor withheat loss compensation in a pulsed mode.

FIG. 10E is a diagram to illustrate temperature and voltage of a Piranisensor over time when used in an electronic circuit as in FIG. 10F.

FIG. 10F shows an alternative electronic circuit for operating a Piranisensor with heat loss compensation in a pulsed mode.

FIG. 11A shows the diagram of FIG. 10A using an alternative control.

FIG. 11B shows the diagram of FIG. 10B using an alternative control.

FIG. 11C shows the diagram of FIG. 10C using an alternative control.

FIG. 12 shows a top view of a micro-Pirani sensor chip.

FIG. 13 shows a top view of a sensor chip with meander-shaped sensingand heating elements.

DETAILED DESCRIPTION

Referring to FIG. 1A and FIG. 1B, a sectional side and top view of aconventional Pirani vacuum sensor are shown. A measuring element 11 issuspended within a fluid. The measuring element 11 is a filament made ofcoiled metal wire. The measuring element 11 is held on both ends bysuspension pins 12, which are electrically conductive. The suspensionpins 12 reach through insulated bushings 14 in a base plate 13. Thelower ends of the suspensions pins 12 reach outside the sensor and areused to electrically connect the measuring element 11 to an electriccircuit. A heat sink 15 is positioned adjacent to the measuring element11. The heat sink 15 may serve as a sensor cover.

During operation the base plate 13 and the suspension pins 12 assumeambient temperature T_(A). Through externally provided power themeasuring element 11 is heated to a controlled temperature T₁. Themeasuring element 11 transfers heat into the surrounding fluid bythermal conduction. Fluid heat transfer 18 from the measuring element 11to the heat sink 15 and the base plate 13 is illustrated by thinstraight arrows. Heat transfer into the fluid is a signal, which can beused to determine characteristics of the fluid, e.g. the fluid'spressure, flow rate or composition. The measuring element 11 alsoconductively transfers heat into suspension pins 12. The suspension heatloss 16 is illustrated by bold arrows. The measuring element 11 furthertransfers heat into the cover 15 and the base plate 13 by radiation.This radiation heat loss 17 is indicated by wavy arrows.

Referring now to FIG. 2A and FIG. 2B a sectional side and top view of aconventional micro-Pirani vacuum sensor are shown. A thin micromachinedmembrane measuring element 21 is suspended in a fluid. The membranemeasuring element 21 is connected by electrically and thermallyconductive suspension leads 22 to a micro-Pirani chip 26. Themicro-Pirani chip 26 is disposed on base plate 23. The membranemeasuring element 21 is electrically connected to leads which leadthrough bushings 24 in the base plate 23. The micro-Pirani sensor isenclosed by a cover 25. The cover 25 has an opening 20 allowing fluid toenter and exit the sensor.

During operation the base plate 23 and the cover 25 assume ambienttemperature T_(A). Through externally provided power the membranemeasuring element 21 is heated to a controlled temperature T₁. Themembrane measuring element 21 transfers heat into the surrounding fluidby thermal conduction. Fluid heat transfer 28 is illustrated by thinstraight arrows. Heat transfer into the fluid is a signal, which can beused to determine characteristics of the fluid, e.g. the fluid'spressure, flow rate or composition. The membrane measuring element 21also conductively transfers heat into suspension leads 22. Suspensionheat loss 26 a is illustrated by bold arrows. The membrane measuringelement 21 further radiates heat into the cover 25 and the base plate23. Radiation heat loss 27 is indicated by wavy arrows.

The prior art sensors of FIGS. 1 and 2, when used as a Pirani vacuumsensors, have a zero pressure corresponding to the sum of the suspensionheat losses 16, 26 a and the radiation heat losses 17, 27. Typically,the conductive heat loss 26 a of a micro-Pirani sensor is lower than theconductive heat loss 16 of a larger Pirani sensor, and correspondinglythe zero pressure of a micro-Pirani sensor as shown in FIG. 2 is lowerthan that of a larger Pirani sensor as shown in FIG. 1. The same appliesto the base power when the sensors are used as fluid flow sensors.

FIG. 3A and FIG. 3B show an improved version of the Pirani sensor as inFIG. 1A and FIG. 1B. A measuring element 31 is suspended within a fluid.The measuring element 31 is a filament made of coiled metal wire. Themeasuring element 31 is held on both ends by suspension pins 32, whichare electrically conductive. The suspension pins 32 reach throughinsulated bushings 34 in a base plate 33. The lower ends of thesuspensions pins 32 reach outside the sensor and are used toelectrically connect the measuring element 31 to an electronic circuit.Positioned adjacent to the measuring element 31 is a heat sink 35. Theheat sink 35 may be a cover which encloses the sensor and also acts as aheat exchange surface.

A heating element 39 a is disposed underneath the measuring element 31and thermally conductively connected to the suspension pins 32. Theheating element 39 a is electrically connected to electrical terminals39 which reach through bushings 34 in the base plate 33. The heatingelement 39 a may dual-function as a temperature sensor to measure thetemperature of the suspension pins 32 and, for example, be a resistancethermometer (Pt100, Pt1000, Ni100, Ni1000).

During operation the base plate 33 and the lower ends of the suspensionpins 32 assume ambient temperature T_(A). Through externally providedpower the measuring element 31 is heated to a controlled temperatureT_(F). The measuring element 31 transfers heat into the surroundingfluid by thermal conduction. Fluid heat transfer 38 is illustrated bythin straight arrows. Heat transfer through the fluid occurs primarilybetween the measuring element 31 and the heat sink 35, which acts as aheat exchange surface. Heat transfer through the fluid is a signal,which can be used to determine characteristics of the fluid, e.g. thefluid's pressure, flow rate or composition. The measuring element 31also conductively transfers heat into the suspension pins 32. Suspensionheat loss 36 a is illustrated by bold arrows. The measuring element 31further transfers heat into the heat sink 35 and the base plate 33 byradiation. Radiation heat loss 37 is indicated by wavy arrows.

Externally provided power to the heating element 39 a through theelectrical terminals 39 provides conductive heat loss compensation 36 bwhich counteracts the conductive heat loss 36 a into the suspension pins32. The externally provided power to the heating element 39 a will alsobe referred to as compensation power (P_(comp)). The compensation poweris selected to at least partially replace the power otherwise dischargedby thermal conduction of the suspension and thus minimize the effect ofthe base power on the actual measurement at the measuring element. Thezero pressure of a Pirani measuring range can be significantly reduced,and the measuring range can be extended towards lower pressures.

A traditional Pirani sensor without heat loss compensation, whenmeasuring a vacuum of 10⁻⁴ mbar, may operate with a measuring elementvoltage of about 300 mV and a measuring element current of about 2 mA.The measuring element hence experiences a total heat transfer of about600 μW. Only 0.1% of this total heat transfer is typically caused byconductive heat transfer through the gas in the vacuum, and 99.9% of theheat is lost through the suspension and through radiation. The lowsignal portion of the total power which is measured limits the operatingrange of a traditional Pirani sensor. By applying compensation power toreduce suspension heat loss, the signal portion of the overall powerapplied to the measuring element can be significantly improved. Thisallows an extended measuring range of down to e.g. 10⁻⁶ mbar. Generallyspeaking, if the suspension is at the same temperature as the measuringelement, no heat flows from the measuring element into the suspension.The suspension may for example be two suspension pins 32, a bracket, orpart of a microchip. Ideally, heat losses at the measuring element 31will occur only through the thermal conductivity of the surroundingfluid and through radiation losses, primarily in the direction of theheat sink 35.

Variation of a fluid's environmental conditions primarily influencesheat flow through the suspension, and to a much lesser extent radiationlosses. The ability to control and reduce suspension heat losses henceprovides a significant improvement. However, when measuring very lowpressures, variable radiation losses and deposits on the surface of ameasuring element can still be disturbing. To minimize also theremaining environmental effect on the measurement noise, radiationlosses can be reduced in three possible ways:

-   -   1. The absolute temperature of the measuring element can be        reduced, thus making it less prone to radiating heat.    -   2. The temperature offset between the measuring element and the        corresponding heat sink may be reduced.    -   3. Highly reflective surfaces may be used.

In a simplified model the measuring element suspension pins 32 and theheating element 39 can be represented by a plate with temperature T_(S)and an area A. The heat sink 35 can be represented as a coplanar plateat temperature T_(A) and area A. Assuming T_(m) is a mean temperaturebetween T_(S) and T_(A) with T_(m)=(T_(S)+T_(A))/2 then the totalradiation loss P_(rad) isP _(rad)=4AEσT _(m) ³(T _(S) −T _(A)),wherein E=radiation exchange factor and σ=Boltzmann constant.

The radiation exchange factor E is composed of the emission coefficientand the reflection coefficient of the radiating and absorbing elementsin the arrangement. E depends on the materials used and their surface.Highly reflective surfaces reduce E and thereby the entire exchangedradiation power.

If the offset between T_(S) and T_(A) decreases, so does the radiationloss. A reduction of the mean temperature T_(m) has a significanteffect, since the radiation loss decreases with the cube of the absolutetemperature T_(m). The radiation loss portion of the total signal of aPirani sensor operating at 400 K is about 3 times higher than if themeasuring element is regulated at e.g. 350 K at the same ambienttemperature of 20° C.

The disclosed arrangement allows an operation with the measuring elementbeing heated to about 353.1K (80° C.). If the adjacent transfer surface(heat sink) is controlled to a temperature of 333.1K (60° C.), thedifference T_(S)−T_(A)=20K, and thus considerably smaller than typicalPirani sensors according to the prior art. Temperature control of theheat sink will be described in more detail below. A difference of onlyabout 20K between T_(S) and T_(A) reduces the desirable heat transferthrough the fluid which is the signal to be measured. However, theassociated radiation power loss decreases faster than the desiredsignal, leading to an improved signal to noise ratio. By lowering theoperating temperature of the measuring element, and reducing thetemperature offset between the measuring element and the surroundingheat exchange surface (heat sink) as described, the signal to noiseration can be improved by approximately a factor of 5.

Further improvements are possible by choosing a highly reflectivesurface for the measuring element 31, the suspension pins 32, and theheat sink 35. The reflective surfaces reduce radiation exchange factor Eand correspondingly the radiation losses.

The shape of the heat sink 35 is influenced by its functions as a heatexchange surface and cover to protect the measuring element 31 fromdamage and contamination. Due to the need of fluid (gas or liquid)exchange, the heat sink 35 may only inadequately protect the measuringelement 31 against contamination. Nevertheless occurring contaminationof the measuring element surface, for example due to condensation, canbe partially eliminated by short-term heating of the measuring element31 to a higher target temperature.

FIG. 4A and FIG. 4B show an improved version of the Pirani sensor as inFIG. 2A and FIG. 2B. A thin micromachined membrane measuring element 41is suspended in a fluid. The membrane measuring element 41 is connectedby electrically and thermally conductive suspension leads 42 to amicro-Pirani chip 46. The micro-Pirani chip 46 is disposed on a baseplate 43. The membrane measuring element 41 is electrically connected toleads 41 a which lead through bushings 44 in the base plate 43. Themicro-Pirani sensor is enclosed by a cover 45. The cover 45 has anopening 40 allowing fluid to enter and exit the sensor. The cover 45also functions as a heat sink.

Heating elements 49 a are disposed on the suspension leads 42 andelectrically connected to terminals 49 which reach through bushings 44in the base plate 43. As shown, one heating element 49 a is placed oneach of the two suspensions leads 42 that hold the membrane measuringelement 41. Both heating elements 49 a are internally wired in parallelto terminals 49. Alternatively, both heating elements could be wired inseries.

During operation the base plate 43, the micro-Pirani chip 46 and thecover 45 assume ambient temperature T_(A). Through externally providedpower the membrane measuring element 41 is heated to a controlledtemperature T₁. The membrane measuring element 41 transfers heat intothe surrounding fluid by thermal conduction. Fluid heat transfer 48 isillustrated by thin straight arrows. Heat transfer into the fluid is asignal, which can be used to determine characteristics of the fluid,e.g. the fluid's pressure. The membrane measuring element 41 alsoconductively transfers heat into the suspension leads 42. Suspensionheat loss 46 a is illustrated by bold arrows. The membrane measuringelement 41 further radiates heat into the cover 45 and the base plate43. Radiation heat loss 47 is indicated by wavy arrows.

Externally provided power to the heating elements 49 a through theelectrical terminals 49 provides heat loss compensation 46 b whichcounteracts the conductive heat loss 46 a. In sum, no conductive heat islost through suspension leads 42.

FIGS. 5A-C show an alternative embodiment of a Pirani sensor with heatloss compensation. Here, a measuring element 51 is suspended by twosuspension members 52. Each suspension member 52 comprises a band-shapedbody, which is arranged in a plane parallel to the base plate 53 of thesensor. One end of the band-shaped body is bent upward. The measuringelement 51 is attached to the upwardly facing end of the suspensionmember and extends above and substantially in parallel with theband-shaped body. The opposite end of the band-shaped body extendssideways into a narrow electrical lead 52 a, which is bent downward toreach through bushings 54 in the base plate 53. A mirrored suspensionmember 52 connects the opposite end of the measuring element 51. Theband-shaped body of both suspension members 52 is thermally conductivelyconnected on top of a heating element 59 a. A heat sink 55 is alsothermally conductively connected to heating element 59 a. As shown, theheat sink 55 comprises a substantially U-shaped cross section. The topof heat sink 55 extends substantially in parallel to heating element 59a. The sides of heat sink 55 extend downwardly onto heating element 59a, creating a tunnel in the center of which measuring element 51 extendsaxially. The heating element 59 a is electrically connected to terminals59 which reach through bushings 54 in the base plate 53. For operationthis embodiment requires the measuring element 51 to at least inintervals assume a temperature above the temperature of the suspensionmembers 52. Suspension heat loss does occur in this embodiment. However,due to the controlled temperature of the suspension members 52 and theheat sink 55 the suspension losses are constant and hence easier todiscern from the heat transfer through the fluid than in traditionalsensors.

FIGS. 5D-F show an alternative embodiment of a Pirani sensor with heatloss compensation. Here, the measuring element 51 a is formed as ameander-shaped foil. The foil is made of suitable material, for examplenickel, hard nickel or stainless steel.

FIG. 5G shows the metal foil used in the Pirani sensor of FIGS. 5D-F inmore detail. As shown, the measuring element 51 a and its electricalterminals are formed by cutting and bending from a single piece of foil.The foil comprises a central meandering section which serves as themeasuring element 51 a. Vertical sections 52 b provide vertical spacingfor the measuring element 51 a and are bent approximately 90° downwardfrom the central meandering section along a bending line 52 d. Bentalong a second bending line 52 d the body sections 52 a extend parallelto the meandering section inward. Extending outwardly from the bodysections are electrical lead sections 52 c. The ends of electrical leadsections 52 c are bent downward to form electrical terminals which reachthrough bushings 54 in the base plate. The body sections 52 a of thisdesign a thermally conductively connected to the heating element 59 a.

An alternative design of the Pirani sensor as generally shown in FIG. 5is illustrated in FIG. 6A. A heat sink 65 a is thermally conductivelyconnected to a base plate 63 a. Both heat sink 65 a and base plate 63 awill normally assume ambient temperature. The measuring element 61 a maybe a filament such as shown in FIGS. 5A-C or a flat measuring element asshown in FIGS. 5D-G.

FIG. 6B shows an alternative sensor with two heating elements. Asuspension heating element 62 b is provided underneath the measuringelement 61 b as shown before. A heat sink heating element 66 b isprovided above the measuring element 61 b and thermally conductivelyconnected to the heat sink 65 b. The heat sink heating element 66 b isheld by the cover 67 b. This allows control of the temperature of theheat sink 65 b independently of the temperature of the suspensionheating element 62 b. The heat sink heating element 66 b is connected byconnection wires 67 b to pins 68 b which reach through bushings in thebase plate 63 b.

FIG. 6C illustrates an electronic circuit suitable for connecting asensor as in FIG. 6B. The measuring element F1 is connected within aWheatstone bridge circuit with resistors R1, R2 and R3. Preferably, theWheatstone bridge is symmetrical with R1=R2. The voltage across thebridge is amplifier by an operational amplifier OP to level U_(a1) andfed into a control processor μC. U_(a1) is a measure of the heat loss inthe measuring element F1, and can be used to derive a fluidcharacteristic such as fluid vacuum pressure by further processing inthe control processor. Two heating element HS1 and HS2 are provided tocompensate heat losses in the suspension elements. As shown, heatingelements HS1 and HS2 are connected in series to a suspension temperaturecontroller TS. The suspension temperature controller TS is operativelyconnected to the control processor μC or to a potentiometer PS. Thepotentiometer PS is used to calibrate the target temperature of thesuspension heating elements HS1 and HS2. This may be done during initialcalibration after production of the sensor. To calibrate the sensor itis put in a vacuum, preferably of less than 10⁻⁵ mbar, e.g. at 10⁻⁷mbar. The potentiometer PS, or alternatively the control processor μCoutput which is connected to suspension temperature controller TS, isadjusted until amplifier voltage across the bridge Ua1 is near zero.

A heat sink heating element H3 is provided and operatively connected toa heat sink temperature controller T3. The heat sink target temperatureis selected either by adjusting potentiometer P3 or electronicallycontrolled by the control processor μC, which is operatively connectedto the heat sink temperature controller T3.

The circuit as shown in FIG. 6C resembles a four-terminal sensingcircuit for measuring electrical impedance. To improve the accuracy ofmeasuring the impedance of the measuring element F1 it is connectedthrough terminals II and III, while heating elements HS1 and HS2 areseparately connected through terminals I and IV. This allows reducingthe current through the measuring element, and hence improving theaccuracy of its measurement.

FIG. 6D illustrates an electronic circuit suitable for connecting asensor as in FIG. 6B. In contrast to the circuit provided in FIG. 6D,sensor calibration in this example is automatically controlled throughthe control processor μC without manual adjustment of potentiometers. Inthis circuit, the measuring element F1 is within a Wheatstone bridgewith resistors R1, R2 and R3. The Wheatstone bridge is powered by avariable voltage driver UB, which is operatively connected to andcontrolled by the control processor μC. The voltage across the bridge isfed into a comparator K1. The control processor μC adjusts the variablevoltage driver UB until the comparator K1 flips. The level of UB atwhich the comparator K1 flips is processed in the control processor toderive a fluid characteristic of interest.

When used as a vacuum sensor, calibration is achieved by placing thesensor with the measuring element F1 (element 61 b in FIG. 6B), thesuspension heating elements HS1 and HS2 (elements 62 b in FIG. 6B) andthe heat sink heating element 113 (element 66 b in FIG. 6B) into avacuum, preferably of less than 10⁻⁵ mbar, e.g. at 10⁻⁷ mbar. Duringcalibration the control processor μC controls the variable voltagedriver UB to a low value which is just sufficient to create a voltageacross the Wheatstone bridge and bias the comparator K1, but smallenough not to heat the measuring element F1. The control processor thenincreases the suspension temperature by adjusting suspension temperaturecontrol TS until comparator K1 flips. The value to which TS wascontrolled when the comparator K1 flipped is stored in a non-volatilememory within the control processor μC.

The disclosed circuit also allows the control processor μC to controlthe temperature of a heat sink 65 b through a heat sink heating element113. The temperature of the heat sink can be independently controlled bythe heat sink temperature controller T3 which is operatively connectedto and controlled by the control processor. As shown, the return path IVof suspension heating elements HS1 and HS2 may be combined with thereturn path III of the measuring element F1.

FIGS. 7A-C and FIG. 8 show various micro-Pirani sensors, mounted ontoheating elements for suspension heat loss compensating. Morespecifically, FIG. 7A and FIG. 7B show a micro-Pirani chip 72 a with ameasuring element 71 a and bonding pads 73 a. The chip 72 a is mountedon a base plate 74 a, which can be temperature controlled by means of asuspension heating element 75 a. A cover 73 b, which acts as a heatsink, is mounted onto a supporting frame 78 a. A cover heating element79 a is provided underneath the supporting frame 78 a. The measuringelement 71 a, the first suspension element 75 a and cover heatingelement 79 a are all electrically connectable from outside of thesensor. For the connection of the measuring element 71 a, the contactsurfaces 77 a may be connected directly via bonding wires with thebonding pad 73 a. A free standing mounting of the base plate 74 a withthe micro-Pirani chip 72 a allows heating the base plate 74 a andmicro-Pirani chip 72 a to a temperature generally above the ambienttemperature with relatively low heating power.

FIG. 7B shows a side sectional view through a sensor as generallydepicted in FIG. 7A. As illustrated, a suspension heating element 75 b,which may be a resistive wire, is embedded within a base plate 74 b. Ameasuring element 71 b is connected to a chip 72 b, which in turn ismounted onto the base plate 74 b. A cover heating element 79 b isembedded into a supporting frame 78 b. A cover 73 b is thermallyconductively attached to the supporting frame 78 b. The supporting frame78 b is freestanding on pins 76 b, spatially separated above a carrierplate 77 b. The pins 76 b protrude the carrier plate 77 b and allowconnecting an electronic circuit to the measuring element 71 b, to thesuspension heating element 75 b, and the cover heating element 79 b.

This design allows controlling the temperature of the measuring element71 b to be the same as the temperature of the base plate 74 b, therebyproviding compensation of the suspension heat loss. Simultaneously, thecover 73 b, which exchanges heat with the measuring element 71 b throughradiation, may be controlled to a second temperature to contain theradiation heat loss to a constant and predictable value. To furtherreduce the radiation heat losses the measuring element facing surface ofthe base plate 73 b and the inner surface of the cover 73 b may bereflectively coated.

FIG. 7C shows a cross sectional view of an alternative Pirani sensor.Here again, a membrane measuring element 71 c is connected to a firstchip 72 c, which in turn is mounted onto a base plate 74 c. Suspensionheating elements 79 c are disposed on the membrane between the measuringelement 71 c and the suspension points of the membrane to the first chip72 c. The suspension heating elements 79 c and the measuring element 71c are electrically connected by bond wires 78 c from the chip 72 c tobase plate 74 c. Stacked onto the first chip 72 c is a second chip 73 cwhich serves as a cover and heat exchange surface. Embedded into thebase plate 74 c is a second heater element 75 c. Through lead wires 76c, the base plate 74 c is connected to contact pins of carrier plate 77c.

The suspension heating elements 79 c are configured to compensateconductive suspension heat loss from the measuring element 71 c into thefirst chip 72 c. The inner surface of the cover 73 c and thechip-bearing surface of the base plate 74 c may be reflectively coatedor metallized, in order to minimize radiation losses.

FIG. 8 is a sectional view of yet another variation of a micro-Piranisensor with heat loss compensation. A micro-chip Pirani 82 carries asensing element 81. The micro-chip Pirani 82 is mounted on a base plate84. Embedded into the base plate 84 is a heat sink heater 85. The heatsink heater 85 is used to hold the base plate 84 at a constanttemperature. A heat sink 83 is thermally conductively attached to thebase plate 84, and hence assumes the same temperature as the base plate84. Optionally, the inner surface of the heat sink 83 and thechip-bearing surface of the base plate 84 can be reflectively coated inorder to minimize radiation losses. Since the base plate 84, themeasuring element 81, and the heat sink 83 are at the same temperature,this structure is preferably operated with pulsed operating method.

The micro-chip Pirani 82 with the measuring element 81 is connectedthrough bonding wires 88 to the base plate 84. Via connecting wires 86that are attached to a pad 87, the measuring unit can be incorporatedinto a sensor assembly. For example, the disclosed micro-chip Piranisensor may be combined with diaphragm pressure sensors, cold cathodevacuum sensors, or hot cathode vacuum sensors.

Referring now to FIG. 9A and FIG. 9B a Pirani sensor with heatingelement enclosed by a metal bracket is shown. The measuring element 91 ais shown as a filament, but may also be implemented as a flatmeander-shaped foil. The measuring element 91 a is attached tosuspension elements 92 a, which dual-function as electrical terminalsfor electrically connecting the measuring element 91. Spacer pieces 93 aare disposed above the suspension elements 92 a and separate thesuspension elements from an upper ceramic heating element 95 a above. Alower ceramic heating element 94 a is disposed below the suspensionelements 92 a. The upper and the lower ceramic heating elements 95 a, 94a are connected by electrical wire leads 98 a.

The lower ceramic heating element 94 a, the suspension members 92 a, thespacer pieces 93 a and the upper ceramic heating element 95 a aresandwiched together and held in place by a metal bracket 99 a. The metalbracket 99 a may for example be made of copper and hence be highlythermally conductive to ensure a uniform temperature across the sandwichstructure.

Since the suspension members 92 a are at the same temperature with themeasuring element 91 a and the upper and lower ceramic heating elements94 a, 95 a, this structure is preferably operated with a pulsed method.As shown in FIG. 9B, fluid, that is a gas or liquid which is to bemeasured, can enter and exit the area around the measuring element 91 athrough a media inlet slot 97 b.

FIG. 12 shows the top view of a micro-Pirani sensor with a measuringelement 121. Heating elements 122 are disposed on a membrane between themeasuring element 121 and the suspension points of the membrane to thesurrounding microchip. The heating elements 122 compensate forsuspension heat loss.

An insulating layer, preferably an oxide layer with good thermalconduction, is disposed above both the sensor element 121 and theheating elements 122. Thermopile or thermocouple elements are disposedon this insulating layer. A first set of thermocouples 123/124, 125/126(or one thermopile with n thermocouples) is positioned above the heatingelements 122. A second set of two thermocouples 127/128 (or onethermopile with 2 n thermocouples) is located above the measuringelement 121. The connecting legs of the thermocouples on the membranesuspensions are lead out to connection pads on the chip.

If the heating elements 122 are at a higher temperature than thesurrounding chip, the first set of two thermocouples 123/124, 125/126will generate a first thermal voltage corresponding to the temperaturedifference between heating elements 122 and surrounding chip. If themeasuring element 121 is at a higher temperature than the surroundingchip, the second set of two thermocouples 127/128 will generate a secondthermal voltage corresponding to the temperature difference betweenmeasuring element 121 and surrounding chip.

If the chip is at ambient temperature, control of the measuring elementtemperature and the temperature of the suspension can take place in asimple manner to achieve a fixed distance from the ambient temperatureby an electronic controller which compares the thermocouple voltage witha fixed voltage reference.

The thermocouples 123/124 may be wired in series with the thermocouples125/126, and their combined thermal voltage and compared with thethermal voltage of the two thermocouples 127/128. When the measuringelement 121 and the suspensions have the same temperature, thedifference of the thermal voltages is equal to zero. The temperature ofthe measuring element 121, which is the same as that of the suspensions,can in a simple manner be kept at a fixed offset from the ambienttemperature as a cascade control.

Operating Methods

Different methods can be used to operate the sensors according to theaforementioned embodiments.

Method 1:

Element Temperature/Control Measuring Element Controlled tosubstantially constant temperature T₁ Measuring Element SuspensionControlled to substantially constant temperature T₁ Cover (Heat ExchangeSurface) Uncontrolled ambient temperature T_(A). May optionally bemeasured by a temperature sensor.A sensor operating according to this method may be calibrated in anultra-high vacuum environment. Calibration may for example be achievedthrough use of a Wheatstone bridge, in which the measuring element isthe unknown electrical resistance to be measured. One of the other threeresistors may be adjusted until the bridge voltage is zero. Calibrationmay also be achieved by inserting a variable corrective voltage into theWheatstone bridge by a control processor. The calibration is saved forfuture measurements and corrects for the radiation heat loss whichoccurs even in a complete vacuum.

Method 2:

Element Temperature/Control Measuring Element Controlled by pulses,alternating between a lower temperature T₁ and a higher temperature T₂Measuring Element Suspension Controlled to substantially constanttemperature T₁ Cover (Heat Exchange Surface) Uncontrolled ambienttemperature T_(A)During pulsed operation of the measuring element the heat capacity ofthe measured fluid influences the measured signal. See Heinz Plöchinger,2002, “Fortschritt in der Vakuum-Messtechnik”, Vakuum in Forschung andPraxis, vol. 14, no. 5, pp. 281-283, and W. Jitschin & S. Ludwig, 2004,“Dynamical behaviour of the Piranisensor”, Vacuum, vol. 75, pp. 169-176

Method 3:

Element Temperature/Control Measuring Element Controlled tosubstantially constant temperature T₁ Measuring Element SuspensionControlled to substantially constant temperature T₁ Cover (Heat ExchangeSurface) Controlled to substantially constant temperature T₃, with T₃ <T₁This method may be implemented for example by the electronic circuit asin FIG. 6C.

Method 4:

Element Temperature/Control Measuring Element Controlled by pulses,alternating between a lower temperature T₁ and a higher temperature T₂Measuring Element Suspension Controlled to substantially constanttemperature T₁ Cover (Heat Exchange Surface) Controlled to substantiallyconstant temperature T₃, with T₃ < T₁

Method 5:

Element Temperature/Control Measuring Element Controlled tosubstantially constant temperature T₁ Measuring Element SuspensionControlled to substantially constant temperature T₃, with T₃ < T₁ Cover(Heat Exchange Surface) Controlled to substantially constant temperatureT₃, with T₃ < T₁The operating according to method 5 leads to a constant and hencepredictable suspension heat loss.

Method 6:

Element Temperature/Control Measuring Element Controlled by pulses,alternating between a lower temperature T₁ and a higher temperature T₂Measuring Element Suspension Controlled to substantially constanttemperature T₃, with T₃ < T₁ Cover (Heat Exchange Surface) Controlled tosubstantially constant temperature T₃, with T₃ < T₁

Method 7:

Element Temperature/Control Measuring Element Power is applied until ahigher temperature T₂ is reached. Power is turned off thereafter,allowing measuring element to cool down to equilibrium state in whichmeasuring element, suspension and cover are at the same temperature T₃.Measuring Element Suspension Controlled to constant temperature T₃, withT₃ < T₂ Cover (Heat Exchange Surface) Controlled to constant temperatureT₃, with T₃ < T₂

Method 7 is based on temporarily achieving an equilibrium state A inwhich no heat transfer takes place in the sensor. During thisequilibrium state A the measuring element is not powered and themeasuring element suspension and cover are heated or cooled untilmeasuring element, suspension and cover are all at the same temperature.The measuring element (optionally the suspensions of a micro-Pirani) issupplied with additional energy only in pulses. After removal of thecorresponding amount of heat through the gas (and a constant amount ofheat in each of the suspensions and the radiation), the equilibriumstate is established again.

Method 8:

Element Temperature/Control Measuring Element Controlled by pulses.Power is applied until a higher temperature T₂ is reached. Coolingperiod is a predetermined time t_(w). Measuring Element SuspensionControlled to constant temperature T₃, with T₃ < T₂ Cover (Heat ExchangeSurface) Controlled to constant temperature T₃, with T₃ < T₂Method 8 leads to the measuring element alternating between anintermediate temperature T_(m) which is between T₂ and T₃. The measuringelement is alternately heated until a target temperature T₂ is reached,and cools for a predetermined period of time. This is different frommethod 7, which waits to start a new measurement cycle until theequilibrium state A is reached. Here, a new measuring pulse starts aftera fixed waiting time t_(w).

Method 9:

Element Temperature/Control Measuring Element Controlled by pulses.Power is applied until a higher temperature T₂ is reached. Coolingperiod is a predetermined time t_(w). Measuring Element SuspensionHeated synchronously by pulses to match temperature of the measuringelement. Cover (Heat Exchange Surface) Controlled to substantiallyconstant temperature T₃, with T₃ < T₂Method 9 applies primarily to sensors with low-mass, such as amicro-Pirani. Here the suspension can be synchronously heated with aramp from T₁ to T₂, thereby reducing the influence of the base power andthe zero pressure is further reduced.

Method 10 introduces an improvement of any of the previously describedmethods with pulsed measuring element operation (methods 2, 4, 6, 7, 8and 9). In the previously described methods the measuring element ispowered during a pulse according to a fixed curve, preferably a voltagewhich ramps up with a ramp angle α. According to method 10, the slope ofthe ramp, i.e. its ramp angle α, is adjustable. Adjustment of the rampangle is preferably controlled by a control processor which is used tocontrol the sensor anyway. Adjustment of the ramp angle allows operationof the sensor over a wider measuring range. A slow rising pulse thatpowers the measuring element, i.e. a small ramp angle α, providessufficient resolution in the measurement time also at low pressure. Athigher pressures the increased heat dissipation through the measuredfluid requires more power, and hence a steeper rise ramp to reach thetarget temperature in a short measurement time. Information about thecurrent measurement range can be derived from one or more previousheat-up and/or cooling time periods of the measuring element itself orbe derived from the power requirement of the controlled heating ofsuspensions and heat sink.

Method 11 introduces an improvement over any of the previously describedpulsed methods in that the target temperature T₂ to which the measuringelement is heated during a pulse is adjustable.

According to method 12, both the slope of the rise ramp (angle α), aswell as the target temperature T2 and the subsequent waiting time t_(w)for the cooling period can be adjusted by the control processor in orderto bake out the measuring element at a higher temperature and therebyeliminate contaminants adhering to the measuring element. Whether thiscleaning process is necessary can be determined during the startup ofthe measuring element at atmospheric pressure by comparing the firstheating time t_(m0) from T₁ to T₂ with a value that is stored in thememory of the control processor, and that has been set during factorycalibration. If the measuring element is contaminated, the first warm-uptime t_(m0) is longer than in the original state. This procedure“Measurement element check and bake-out” can optionally be executed bypressing a button or by an external command.

Where the disclosed methods refer to controlling a constant or constanttemperature one skilled in the art will appreciate that no control isperfect, and hence “constant temperature” refers to a temperature thatis at a substantially constant level within what is technicallyachievable. One skilled in the art will also appreciate that variationsof the disclosed methods can achieve similar results. For example, wheretwo temperature values are controlled, not both of them need becontrolled to an absolute value. Rather, one may be controlled tomaintain a predetermined temperature offset from the other. Thetemperature of the suspension elements may for example be maintained ata constant offset from the cover temperature. The cover temperature maybe variable, e.g. uncontrolled at the ambient temperature, at anabsolute temperature, at a predetermined offset from the ambienttemperature, or even following a predetermined curve.

While the described sensors and methods are primarily intended tomeasure pressure of a gas, they can also be used for identification of agas. A particular gas can be identified by pattern-matching if the heatconductivity and, optionally, the heat capacity of the respective gasdepends on the temperature. The gas to be identified is exposed to asensor at a constant pressure, for example at atmospheric pressure. Anyof the methods above may be used to identify the gases heat capacity,which is compared with predetermined values stored in a lookup table.

FIG. 10A shows the temperature over time of a measuring element whenoperated in a pulsed mode. From an ambient temperature level, e.g.T_(A2), the measuring element is heated to a controlled temperaturelevel T_(F1). T_(F1) is the steady temperature to which the suspensionis controlled. After a settling time a first pulse on the measuringelement is started by the control processor at t₀. The voltage which isapplied to the measuring element is illustrated in FIG. 10B. The powerapplied to the measuring element causes it to heat from T_(F1) toT_(F2). By measuring the temperature of the measuring element, andcomparing it with a target value, the control processor stops the pulseat time t₁ when the measuring element has reached temperature T_(F2).

The period of time t_(m0) between t₀ and t₁ is essentially a measure forthe amount of heat was lost by heat conduction through the measuredfluid and thus a measure of the fluid pressure. If this time t_(m0)recorded and stored for an individual brand new sensor, it can be usedlater as a reference for contamination of the sensor element.

If the cover of the sensor is at ambient temperature, e.g. T_(A2), thecooling of the measuring element will take place according to the dashedcurve K_(a2). Dashed cooling curves K_(a1), K_(a3), and K_(a4)respectively illustrate the cooling for alternative ambient temperaturesT_(A1), T_(A3), and T_(A4). However, when the cover temperature is alsocontrolled to T_(F1), a cooling curve A1 applies. Under high vacuum, thetime taken to reach the point A1 is very long.

Provided a substantially constant temperature offset between T_(F1) andT_(F2), the cooling curve is always the same at a given pressure. Thecooling curve of the de-energized measuring element can thus also beused in the measurement. In a simple way this is achieved in that afterswitching off at time t₁ the control processor waits a fixedpredetermined waiting time t_(w) until at the time t₂ a new pulsestarts. The sensing element need not be cooled to the point A1. At thesame pressure the new heating time t_(m1) (between t₂ and t₃) of themeasuring element will shorter than t_(m0). This new heating time t_(m1)not only contains information about the current heat loss by conductionthrough the fluid, but also depends on the previous cooling curve whichcrossed t₂ at the start of the new pulse.

At constant pressure, another pulse after a further waiting time t_(w)at time t₄ will lead to the same heating time: t_(m1)=t_(m2). Anysubsequent heating time will be identical and referred to as t_(mx). Thetime t_(mx) as well as the corresponding frequency (1/t_(mx)+t_(w)) mayserve as a measuring signal. A change in pressure of the fluid leads toa change in time t_(mx) and of the corresponding frequency.

FIG. 10B shows a ramp pulse voltage over time with a pulse-rise angle αapplied to the measuring element to generate heat pulses of themeasuring element as in FIG. 10A.

FIG. 10C shows an alternative voltage pulse over time, to generate heatpulses generally as in FIG. 10A.

FIG. 11A shows the temperature over time of a measuring element whenoperated in a pulsed mode as in FIG. 10A, but using a different heatingpulse shape. As illustrated in FIG. 11B the ramp angle of the voltagepulses applied to the measuring element have a smaller ramp angle α thanthose in FIG. 10B. Correspondingly, all else being equal, time periodst_(m0) and t_(mx) are extended. Generally, the lower a pulse-rise angleα, the better the time resolution at low pressures.

In addition to the rise angle α the target temperature T_(F2) and thewaiting time t_(w) may also adjusted by the control processor to carryout a measuring range adaptation or eliminate dirt adhering to themeasuring element.

If the temperature of the suspension, the temperature of the measuringelement and the surface temperature of the heat sink are the same levelT_(F1), no heat flow takes place. From this equilibrium state power maybe applied to the measuring element during a measuring cycle until themeasuring element reaches a new constant temperature T_(F2). Power maybe applied in form of constant voltage, constant current, ramped voltageor ramped current. While the measuring element is at a temperaturehigher than the surrounding cover surface, heat is conducted through thefluid surrounding the measuring element into the heat sink. The amountof heat conducted through the fluid varies with the pressure of thefluid, and can hence be used to measure fluid pressure. Simultaneouslyheat losses through radiation and heat transfer into the suspensionoccur, but those depend on the known difference between T_(F1) andT_(F2), and are hence constant.

In case power is supplied according to a fixed characteristic curve(pulse, ramp) the initial heating time t_(m0) from start at time t₀ andtemperature T_(F1) to stop at time t₁ and temperature T_(F2) is ameasure of the measured fluid variable, for example its pressure. Afterreaching temperature T_(F2), the measuring element remains without poweror with substantially reduced power and cools down to temperatureT_(F1). The cooling curve is again dependent only on the measured fluidvariable and the temperature delta between T_(F1) and T_(F2).

With the same measured fluid variable, e.g. at constant fluid pressure,the cooling curve always has the same shape. Thus, not only therelationship between measured variable and heating curve, but also therelationship between measured variable and cooling curve can be used formeasuring. The detection of an equilibrium state in which measuringelement, suspension and cover are at the same temperature istheoretically possible, but practically difficult. Also, allowing themeasuring element to cool down all the way to the cover temperatureextends the measurement time, especially when the fluid pressure is low.It is hence more beneficial to not let the measurement element cool downcompletely, but rather apply a fixed cool down time period t_(w). As ofthe second cycle t_(m1) the sensor so operated automatically assumed aconstant cycle frequency, which is a measure of the measured fluidvariable and is largely independent of environmental influences.

If the offset between T_(F1) and T_(F2) is small, sufficiently shortmeasurement times can be achieved. Losses through the suspensions andradiation are present, but they are minor and constant.

FIG. 10D illustrates an electronic circuit suitable for connecting asensor as in FIG. 6B and operating it in a pulsed mode as illustrated inFIG. 10A. The circuit here presented is based on the disclosure of athermocouple vacuum gauge sensors in U.S. Pat. No. 8,047,711 by the sameapplicant which has been incorporated by reference. In contrast to thecircuit provided in FIG. 6D, impedance of the measuring element F1 isevaluated by two separate comparators K1 and K2. The measuring elementF1 forms a voltage divider with series resistor R4. The seriesconnection of R4 and the measuring element are powered by a voltage rampgenerator UR. Connected in parallel to R4 and the measuring element is aseries of three resistors R1, R2 and R3. The voltage ramp generator URis operatively connected to and controlled by a control processor μC.The impedance ratio of the measuring element F1/(F1+R4) is comparedagainst a lower threshold R3/(R1+R2+R3) by a lower threshold comparatorK1. The impedance ratio of the measuring element F1/(F1+R4) is alsocompared against an upper threshold (R2+R3)/(R1+R2+R3) by an upperthreshold comparator K2. Both comparators are operatively connected tothe control processor.

Since the temperature and impedance of the measuring element F1 arecorrelated, the lower threshold comparator K1 provides a signal to thecontrol processor μC when the temperature of the measuring element F1falls below a lower temperature threshold TF1. The upper thresholdcomparator K2 provides a signal when the temperature of the measuringelement F1 rises above an upper temperature threshold TF2.

When used as a vacuum gauge, the pressure of the gas surrounding themeasuring element F1 can be determined as described with reference toFIG. 10A before. After reaching the upper threshold temperature thecontrol processor μC turns off voltage ramp generator UR. After apredetermined wait time t_(w) the control processor enables voltage rampgenerator UR and measures the time t_(x) until K2 again indicatesreaching the upper temperature threshold. The measure t_(x) isindicative of the vacuum pressure and can be further processed by thecontrol processor.

Calibration is achieved by placing the sensor into a vacuum, preferablyof less than 10⁻⁵ mbar, e.g. at 10⁻⁷ mbar. During calibration thecontrol processor μC controls voltage ramp generator UR to a lowconstant value which is sufficient to bias comparator K1, but smallenough not to heat the measuring element F1. The control processor μCthen increases the suspension temperature by adjusting the suspensiontemperature controller TS until the comparator K1 flips. The value towhich the suspension temperature controller TS was controlled when thecomparator K1 flipped is stored in a non-volatile memory within thecontrol processor μC.

The control processor μC also controls the temperature of a heat sinkdisposed adjacent to the measuring element F1 by adjusting power to theheat sink heating element H3 through heat sink temperature controllerT3.

As illustrated in FIG. 10E, instead of applying a predetermined waitperiod t_(w), the sensor may operate by alternating the measuringelement temperature between an intermediate temperature threshold TF2′and an upper temperature threshold TF2. This is achieved by adding anadditional intermediate temperature comparator K2′ to the electroniccircuit as illustrated in FIG. 10F. The control processor μC in thisexample activates the voltage ramp generator UR in response to anintermediate temperature threshold signal from the intermediatethreshold comparator K2′. The control processor deactivates the voltageramp generator UR in response to an upper temperature threshold signalreceived from upper threshold comparator K2.

FIG. 13 shows an alternative sensor, in which measuring element 131 is asubstantially flat meander-shaped wire or foil. The measuring elementhere extends sideways from a base plate 134 into the fluid and isconnected to electrical terminals 137. The base plate 134 is heated bysuspension heating element 133, which is a meander-shaped heating wiredisposed in the same plane as the measuring element. The wires betweenthe electrical terminals 137 to the measuring element 131 are thermallyconnected to the base pate 134 to pick up the applied compensation powerfrom the heating element 131.

While the present invention has been described with reference toexemplary embodiments, it will be readily apparent to those skilled inthe art that the invention is not limited to the disclosed orillustrated embodiments but, on the contrary, is intended to covernumerous other modifications, substitutions, variations and broadequivalent arrangements that are included within the spirit and scope ofthe following claims.

What is claimed is:
 1. A method for measuring a characteristic of afluid, comprising: suspending a measuring element within the fluid, themeasuring element being held by suspension members; detecting an uppertemperature threshold of the measuring element; applying measuring powerto the measuring element during a heating phase until the measuringelement reaches the upper temperature threshold; applying compensationpower to one or more suspension heating elements which are thermallyconductively connected to the suspension members; evaluating heattransfer from the measuring element into the fluid; and deriving thecharacteristic of the fluid, wherein the compensation power is selectedto at least partially compensate parasitic conductive heat loss from themeasuring element into the suspension members, wherein evaluating theheat transfer from the measuring element into the fluid comprisesmeasuring a time t_(x) required to heat the measuring element from afirst temperature T₁ to a second temperature T₂, wherein following theapplication of measuring power during the heating phase a predeterminedcooling phase wait period t_(w) is applied during which measuring poweris turned off or substantially reduced, and wherein at least one of thefrequency 1/(t_(x)+t_(w)), the sum of t_(x)+t_(w), or the ratiot_(x)/t_(w) is used to derive a pressure of the fluid.
 2. A method formeasuring a characteristic of a fluid, comprising: suspending ameasuring element within the fluid, the measuring element being held bysuspension members; detecting an intermediate temperature threshold ofthe measuring element; detecting an upper temperature threshold of themeasuring element; applying measuring power to the measuring elementduring a heating phase until the measuring element reaches the uppertemperature threshold; applying compensation power to one or moresuspension heating elements which are thermally conductively connectedto the suspension members; evaluating heat transfer from the measuringelement into the fluid; and deriving the characteristic of the fluid,wherein the compensation power is selected to at least partiallycompensate parasitic conductive heat loss from the measuring elementinto the suspension members, wherein following the application ofmeasuring power during the heating phase measuring power is turned offor substantially reduced during a cooling phase until the measuringelement reaches the intermediate temperature threshold, and wherein aduration of the heating phase or a duration of the cooling phase or asum thereof is used to evaluate heat transfer from the measuring elementinto the fluid.
 3. A method for measuring a fluid characteristic,comprising: suspending a measuring element within the fluid, themeasuring element being held by a suspension member; applying measuringpower to the measuring element during a heating phase until themeasuring element reaches an upper temperature threshold; turning off orsubstantially reducing the measuring power during a cooling phasesubsequent to the heating phase, until the measuring element reaches anintermediate temperature threshold; applying compensation power to oneor more suspension heating elements which are thermally conductivelyconnected to the suspension member; and evaluating heat transfer fromthe measuring element into the fluid by determining a duration of theheating phase or a duration of the cooling phase or a sum thereof. 4.The method as in claim 3 wherein the compensation power is selected toat least partially compensate parasitic conductive heat loss from themeasuring element into the suspension members.
 5. The method as in claim3, wherein the fluid characteristic is a vacuum pressure.
 6. The methodas in claim 3, further comprising the step of exposing the measuringelement to vacuum at or below a lower sensing range and calibrating thecompensation power.
 7. The method as in claim 3, wherein measuring poweris applied in pulses.
 8. The method as in claim 3, further comprisingcontrolling the temperature of the measuring element and controlling thetemperature of the one or more suspension heating elements.
 9. Themethod as in claim 8, further comprising controlling the temperature ofa heat exchange surface which is positioned near the measuring element.